A methanation reaction is a catalytic reaction of hydrogen with carbon monoxide and/or carbon dioxide to produce a methane-rich gas. This methane-rich gas is sometimes also referred to as synthetic natural gas (SNG) and can be used as substitute gas for natural gas. In areas where there is little natural gas available, other sources of energy, such as coal or petroleum coke, may be partially oxidized in a gasification process to produce a gas comprising hydrogen and carbon monoxide. Such a gas comprising hydrogen and carbon monoxide is sometimes also referred to as synthesis gas. The synthesis gas can subsequently be used to produce synthetic natural gas (SNG) in a methanation process.
The methanation reaction proceeds, in the presence of a suitable methanation catalyst, in accordance with the following equations:CO+3H2═CH4+H2O+heat  (1)CO2+4H2═CH4+2H2O+heat  (2).
The water formed during the reaction can, depending on the catalyst, temperature and concentrations present, subsequently react in-situ with carbon monoxide present in a water-gas shift reaction in accordance with the following equation:CO+H2O═CO2+H2+heat  (3)
Reaction (1) is considered the main reaction and reactions (2) and (3) are considered to be side reactions. All the reactions are exothermic.
The methanation reaction can be carried out in one or more adiabatic reactors. As only a partial conversion may be achieved in one adiabatic reactor, conventionally a series of adiabatic reactors is used in a methanation process. As the methanation reaction is exothermic, the temperature of a reaction mixture will increase during passage through the adiabatic reactors. The methanation reactions are reversible and an increasing temperature will tend to shift the equilibrium towards a lower yield. When a series of adiabatic reactors is used, the effluent of an adiabatic reactor is therefore cooled before entering a subsequent adiabatic reactor, for example by using external heat exchangers. In addition, the temperature increase in a first adiabatic reactor is conventionally limited by diluting a feed gas entering the first adiabatic reactor with a methane-rich gas. For this purpose a considerable portion of methane-rich product gas generated in the first adiabatic reactor is cooled and recycled. For example, a feed gas to a first adiabatic reactor may be mixed with recycled methane-rich gas in a volume ratio of recycled methane-rich gas to feed gas as high as about 6:1.
Due to this large recycle stream, a large volume of gas needs to be processed through the first adiabatic reactor. As a consequence such a first adiabatic reactor conventionally has a large volume that may be as high as about 600 or 700 cubic meters. In addition the compressor load for any compressor used to compress the recycled methane-rich gas is high.
An example of a conventional methanation process is provided in the report titled “Haldor Topsøe's Recycle Energy-efficient methanation process” which is available from the website of Haldor Topsøe, www.topsoe.com. In the methanation process illustrated on page 4 of the report a feed comprising hydrogen and carbon monoxide is fed to a series of three adiabatic reactors. After each adiabatic reactor the reactor effluent is cooled in a heat exchanger and part of the reactor effluent of the first adiabatic reactor is cooled, recycled and mixed with the feed gas.
GB2018818 describes a process for preparing a methane-rich gas in at least one adiabatically operating methanation reactor by converting a combination of a preheated synthesis gas stream and a recycle stream from the methanation reactor. The combined preheated synthesis gas stream and recycle stream are passed through a layer of shift catalyst directly before passage through a methanation catalyst. The process of GB2018818 is illustrated with three experiments. GB2018818 states that because of the limitations of the used compressor and in contradistinction with the intended industrial operation the outlet stream of the reactor in these experiments was cooled to below 100° C. According to GB2018818, hereby all the steam was condensed out, whereafter the dry outlet stream was divided into a recycle stream and a product stream. After compression of the recycle stream and before feeding of the recycle stream into the reactor a calculated amount of water was added to the recycle stream to compensate for the removed water. The volumetric ratio of the recycle stream to the synthesis gas stream in the experiments was in the range of 2:1 to 3:1. The volumetric ratio of the recycle stream to the product stream was in the range from 4:1 to 5:1.
U.S. Pat. No. 4,235,044 describes a process for the methanation of a synthesis gas wherein a synthesis gas stream is divided into two separate processing streams. A first stream is reacted with steam in a water gas shift zone to produce a converted gas stream containing carbon dioxide and hydrogen. A first portion of the second unconverted stream is added to the converted gas stream to prepare an adjusted gas stream that is adiabatically reacted in a first adiabatic reaction zone to form an effluent gas stream containing methane. The effluent gas stream from the first adiabatic reaction zone is cooled and mixed with the remaining portion of the second unconverted stream to prepare a reaction mixture that is passed to an isothermal methanation zone or to a second adiabatic methanation zone and subsequently to an isothermal methanation zone. Carbon dioxide can be removed from the product methane-rich gas or from the methanation feed gas. The process uses no recycle. In between methanation zones no carbon dioxide is removed.
U.S. Pat. No. 3,904,389 describes a process for the production of a methane-rich gas from a gaseous effluent of a fossil fuel gasification wherein the gaseous effluent is divided into two fractions. The first effluent fraction is subjected to methanation. The second effluent fraction is successively subjected to shift conversion and CO2 removal. Hereafter the resulting effluents are mixed again and subjected to another methanation. The process uses no recycle. In between methanation zones no carbon dioxide is removed.
It would be an advancement in the art to provide a methanation process that allows an adiabatic reactor to be sufficiently cooled with a small recycle stream and/or at a low ratio of recycled methane-rich gas to feed gas.